By Cat Holmes
University of Georgia
When a single cow tested positive May 20 for Bovine Spongiform
Encephalopathy (BSE) or “mad cow disease” in northern Alberta,
Canada, the U.S. Department of Agriculture went on high alert.
It temporarily closed U.S. borders to Canadian beef and related
products, including animal feed.
That’s one of the reasons BSE, a disease that causes fatal
brain degeneration in cattle, has never been found in the
United States, said Ronnie Silcox, an animal scientist with the
University of Georgia College of Agricultural and Environmental
Sciences.
“Since 1989, the U.S. has banned imports of live animals or
animal products from countries that are at risk for BSE. So
closing our borders to Canada beef was par for the course,”
Silcox said. “We know how BSE is spread, and we’ve put a number
of systems in place to keep it out of our country.”
This wasn’t the case when BSE first appeared in Great Britain
in 1986.
“Then, it was a strange new disease, with an unknown cause or
source,” Silcox said. “It took several years to see the link
between using animal byproducts in animal feed and the
disease.”
During that time, the numbers of BSE-infected cattle in the
United Kingdom skyrocketed, though not in the way contagious
diseases do.
Unlike other feared cattle diseases like foot-and-mouth, BSE
isn’t contagious. “It’s not transmitted by contact with other
infected animals,” Silcox said. “It’s not caused by a virus or
bacteria. It’s spread by feeding byproducts from infected
cattle to other cattle.”
It can take two to eight years after a cow eats infected feeds
for signs of BSE to show up, he said.
Since 1997, the United States and Canada haven’t allowed
protein from cattle, sheep, goats, bison, elk or deer, animals
also known as ruminants, to be fed to other ruminants.
“This ‘animal feed rule’ eliminates the only cause of BSE, as
far as we know,” Silcox said. “Banning byproducts like meat and
bone meal in cattle feed ensures that we’re not feeding
products that could spread the disease even if it were
discovered.”
Indeed, investigators from the Canadian Food Inspection Agency
(CFIA) have found that the BSE-infected cow didn’t enter
Canada’s food supply. The cow’s remains were rendered and may
have been used for the manufacture of a dry dog food. Dogs
aren’t at risk for BSE or any similar disease.
BSE belongs to a family of diseases known as transmissible
spongiform encephalopathies, which include chronic wasting
disease, transmissible mink encephalopathy and variant
Cruetzfeldt-Jakob disease (vCJD). All are fatal. And the
latter, vCJD, effects humans.
The Centers for Disease Control and Prevention Web site notes
that as of April 2, 2002, 125 cases of vCJD had been reported
in the world, most in the United Kingdom. Evidence strongly
suggests that vCJD is caused by eating meat from BSE-infected
cattle.
“CJD is like lightning striking — it doesn’t happen often but
that’s not much comfort if you’re hit,” Silcox said. “It’s
fairly rare even in countries where there was a lot of BSE
infection.”
Of course, the question remains as to how Canada’s BSE-infected
cow got it in the first place. According to the CFIA Web site,
the BSE-infected cow, which died in January, was born before
the 1997 animal feed rule. It could have contracted the disease
before then.
Several other theories are under investigation.
(Cat Holmes is a science writer with the University of Georgia
College of Agricultural and Environmental Sciences.)
